This is one of two things. Either you have powdery mildew or downy mildew, pray you don't have downy mildew ( although thats what it looks like ) because it is harder to manage and can destroy plants in a short matter of time. Here's what you do... research these forums for tips on mildew treatments first, then decide your plan of attack. I started with a skim milk spray of 9 parts to 1 ( 1 part skim milk ) sparayed them in dark time. That helped but didn't take care of it totally so I went and bought a fungicide called DEFENDER, after using this stuff the issue was gone for good. Another tip is take everything out of the grow space and clean EVERY FUCKING SQUARE INCH of space and equipment, I mean be thorough! Also make sure that your air quality is good, example; if you have a passive intake coming from a questionable source tape cheescloth around your ducting to prevent spores from entering space.
Cleanliness is everything. Hope this helps you beat this disease, you should be able to, you caught it early. Good luck